A motor's torque/speed characteristics can be the most important factors when selecting a motor for a specific application. In both open-loop and closed-loop (servo) applications, the loading condition on a motor has critical effects on its dynamic response, efficiency, and power consumption, and the effect varies widely at different speeds. FIG. 1 shows an example of a torque-speed curve that is available for a typical brushless DC motor (24V, 50 oz-in NT Dynamo™ model, Hurst Manufacturing™). Information is also provided on current, efficiency and power output vs. torque.
The existing motor performance charts in the datasheets that manufacturers provide to potential customers are based on benchmark testing at a steady state. Not every possible torque load is actually tested to generate the datasheet charts. Instead, certain loads are applied to the motor to determine the corresponding performance, and the results of the benchmarking are curve-fitted to generate the torque-speed characteristic curve.
The Handbook of Electrical Motors (H. A. Toliyat & G. B. Kliman, Handbook of Electric Motors, Marcel Dekker Inc., New York, 2004) recommends that the user collect at least 7 data points reasonably spread out from no-load to 1.5-2 times the rated load. This information is made available to the customer for selecting the most appropriate motor for their specific application. Because the data points are fitted, the torque-speed combination read from the curve may not correspond exactly to the actual performance of the motor, even for a speed or torque value that was tested. A customer may require a more precise reading for the motor performance under the specific conditions it will operate at, as well as the motor's transient dynamics at the start-up or response to dynamic loads in order to select the most suitable motor for the application. For generating the information for a motor datasheet, the conditions under which the tests are to be carried out are very limited. For example, according to IEEE Std 113-1985 (IEEE Std 113-1985, IEEE Guide: Test Procedures for Direct-Current Machines), which provides guidelines for carrying out these tests, the tests should be carried out with the motor “hot” from continuous operation at the motor's rated load. In addition, the load should be gradually applied and removed until the results are consistent. The curve-fit should have an R2 value of at least 0.9999 and individual data points should be off by no more than 0.25% from the curve. If these two conditions are not met, data points that vary the most from the curve should be retested. Therefore, the torque-speed curve presented in the datasheet does not give adequate information on the repeatability and variability of the motor performance under various load systems.
Some existing systems rely on computer simulation for providing the additional data points. Computer simulation, however, is a computation solution based on a theoretical model of a system. It would be more beneficial to provide an emulation based model. Hardware-in-the-Loop (HIL) simulations enhance purely computer based simulation by having actual components (as opposed to just models) providing information on their performance. Often these are the components that are hardest to model. Emulation is one of the tools available for performing HIL simulation.
There are also issues with coupling an emulation system to a motor under test. Prior Art coupling mechanisms, such as the multi jawed coupling shown in FIG. 2 are based on a toothed positive contact clutch. However, multi jawed couplings do not have a means for ensuring concentricity of the two shafts. In other words, there is no means to correct parallel or angular misalignment. These couplings are also severely limited in size; they cannot accommodate a shaft bore greater than ¼ inch. In addition, positive contact clutches do not have a means of orienting the two hubs to facilitate safe engagement. They are engaged while rotating, at speeds up to 300 rpm.
There exist several generic automatic couplings, some used in the context of a test bed. Dynamic load test systems are found mostly in academic literature and not in practical applications. Dynamic dynamometers have been used first to evaluate combustion engines, and more recently to evaluate electric motors. Dynamometers dissipate the power from a test motor by transferring it to a generator or by using a brake, such as a magnetic particle brake or eddy current brake. Due to their physics, dynamometers are heavy and bulky and cannot emulate fast dynamics. Thus, they are usually used for large-size motors and in situations where the high costs of their acquisition, installation, and maintenance can be afforded. A dynamic dynamometer is proposed in “Emulating Dynamic Load Characteristics Using an Dynamic Dynamometer” (R. W. Newton, R. E. Betz and H. B. Penfold, “Emulating Dynamic Load Characteristics Using an Dynamic Dynamometer”, Proceedings of the IEEE Power Electronics and Drive Systems Conference, Singapore, February, 1995, pp. 465-470), which utilizes a DC motor to provide an active load. A block diagram of this system is shown in FIG. 3. The Experimental Machine for which dynamic performance characteristics are required is connected to the Load Machine via a torque transducer, though the torque transducer may be omitted if one has complete knowledge of the Experimental Machine.
Similar setups are found in U.S. Pat. No. 5,823,104 to Suga and U.S. Pat. No. 3,898,875 to Knoop et al. These are designed for testing the performance of electric motor for drive systems of automobiles during acceleration and deceleration.
U.S. Pat. No. 4,807,487 to Kugler, meanwhile, is for testing drive systems of vehicles using inertia masses. This system is for testing all components of the drive system, not just the rotational power source.
U.S. Pat. No. 6,539,782 to Drecq is for a load machine for a combustion engine test bench utilizing an eddy current brake and a reluctance motor. For combustion engines, in addition to applying an active resistive load, the motor needs to drive the motor at 40% of the full load in order to test the engine braking capabilities of the engine.
Automation Technologies Inc. has developed a system, called Digitorque™, for testing DC motors as they come off a production line to ensure quality control. They test motors for locked-rotor current and torque, pull-up torque, breakdown torque and speed, and full load speed, current and power. A simplified model of the Digitorque architecture is shown in FIG. 4. The results are compared to the characteristics of a “master motor” to ensure that they are within the acceptable limits. However, because each motor has to be tested as it comes off the production line at production speed, the Digitorque system requires an automated means of coupling test motors coming off the production line to the test fixture. Three solutions are offered depending on the size and type of the test motor. For small, low-power motors, a test fixture assembly consisting of a centering mechanism (not described), a collet for clamping to the motor shaft, a shaft flywheel and an encoder is used. For motors such as automotive-type DC starters with a gear end, the motors are mounted on a pallet during assembly, gear down, and the fixture assembly with mating gear was moved to engage with the test motor. For pump motors, a coupling using two unidirectional clutch bearing is used, as shown in FIG. 5. However, while these solutions do allow for automated coupling, they are specific for the motor being tested.
Therefore, there is a need for a system that allows motor manufacturers and vendors to offer their customers the ability to test available motors themselves under the specific load conditions the motor will ultimately be subjected to in its real-world application while the customers are making decisions about the suitable motor to acquire.